Catheters are used in various medical applications, including the diagnosis, treatment and monitoring of medical conditions. One type of catheter includes a sensor for in vivo measurement of various physiological pressures. An illustrative application for a catheter-based pressure sensor is in the measurement of intracranial pressure in which one end of the catheter is disposed in the ventricular region of the brain and the other end exits the cranial region for coupling to a monitor.
Typical catheter pressure sensors utilize a pressure sensitive device, which may be capacitive, resistive, optical or ultrasonic. Generally, one side, or surface of the sensor is exposed to the physiological pressure to be measured and another surface of the sensor (i.e., the reference surface) is exposed to a liquid or gas at a reference pressure. The measured pressure differential between the sensor surfaces provides an indication of the physiological pressure to which the first surface is exposed.
As with most electronic sensing devices, it is advantageous to calibrate the catheter pressure sensor periodically to compensate for sensor drift at a zero pressure condition. Certain medical conditions result in an increase in intracranial pressure (e.g., due to production of excess cerebrospinal fluid). Pressures beyond a certain threshold can cause patient death or irreversible brain damage. It is thus necessary to obtain reliable pressure readings.
Calibration is typically achieved by applying a known, equal pressure to both surfaces of the sensor and measuring an output signal of the pressure sensor. The measured output signal represents a zero offset of the sensor and the process of calibration is thus sometimes referred to as "zeroing. " Often, the known pressure at which the sensor is calibrated is atmospheric pressure. The measured zero offset of the sensor is used by a circuit to adjust the measured signal in order to compensate for the zero offset, thereby enhancing the pressure sensing accuracy of the device.
Some catheter pressure sensors permit calibration to be performed in vivo. One such catheter is described in U.S. Pat. No. 4,901,735 (von Berg) in which a balloon structure surrounds the entire catheter tip, where the sensor is located, and is inflated when calibration is performed. With the balloon inflated, a uniform pressure is exerted on both surfaces of a strain gauge sensor. Another catheter pressure sensor permitting in vivo zeroing is described in U.S. Pat. No. 5,203,340 (Gustafson et al.) in which an ex vivo pressure connecting means is provided for inhibiting liquid communication with the reference surface of the sensor and permitting the physiological pressure to be applied to both surfaces of the sensor. Although some pressure monitors are able to achieve in vivo zeroing, more reliable and effective calibration of in vivo monitors would be desirable.